Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Relay node (RN) is a kind of entity which exists between DeNB and UE. The architecture for a relay node has been proposed in Reference [1] (3GPP TS 36.806 V9.0.0, March, 2010).
Because high speed public transportation is being deployed worldwide at an increased pace, the requirement for Mobile Relay is more and more eager, for example, referring to Reference [2] (RP-110894, 3GPP TSG RAN#52, May 31-Jun. 3, 2011).
According to definition for mobile relay in Reference [2], one of mobile relay's major characters is the capacity to support group mobility. The reason to support group mobility is that excessive handover signaling can be avoided by performing a group mobility procedure instead of individual mobility procedures for every UE, so handover success rate can be improved via mobile relays capable of group mobility.
One group mobility procedure in LTE system may include the following phases:
1. Trigger of handover                a. Terminal (or relay node) send Measurement Report to base station (for example, eNB);        
2. Preparation of handover                a. Signaling messages are exchanged between source base station and target base station via X2/S1 interface (for S1 handover, core network are also involved in preparation phase);        b. New UE context for each terminal is created in target base station based on the exchanged signaling messages;        
3. Execution of handover                a. Radio interface for each terminal is reconfigured so that to align UE's radio configuration with target base station's radio configuration, wherein the radio configuration includes at least one of the following:                    i. New measurement configuration in target base station;            ii. Mobility control information;            iii. UE dedicated Radio resource configuration;            iv. Security configuration;                        b. Each terminal triggers random access and achieves uplink synchronization with new serving base station (in group mobility, from the point of UE, new serving base station is still the relay node);        c. Radio network changes backhaul transmission for the UE from source base station to target base station.        
After mobile relay is introduced, each LTE terminal held by passengers on one vehicle (e.g., one high speed train) is connected with the mobile relay node mounted on the vehicle, and the mobile relay node in turn relays the communications between each individual terminal and on-land base station. When the vehicle moves across edge of two macro cells, the mobile relay node's backhaul radio link will change from source DeNB to target DeNB, but each UE on the vehicle still connects with the relay cell served by the mobile relay node. So during the group mobility (i.e., Relay Node mobility), for the following reasons, it is not very necessary to reconfigure radio interface configuration for each individual UE connected with the relay node on the vehicle:                1. LTE supports detected cells, so it is not necessary to list all cells need to be monitored within one measurement object. So during relay node handover (group mobility), measurement configuration for each UE does not need to be changed;        2. After relay node mobility, UE is still connected with the relay cell served by the mobile relay node, so it is not needed to send UE dedicated Radio resource configuration/Mobility Control Information to UE.        
The sole radio configuration that needs be synchronized between UE and DeNB is security configuration. Because the value range of NCC (Next Hop Chaining Counter) is limited, from 0 to 7, if mobile relay node always ignores sending the securityConfigHO to UE during Relay Node handover, then after the 8th inter-eNB Relay Node handover, the value of the NCC maintained in DeNB will round up, thus DeNB and UE cannot maintain synchronized (NH, NCC) pair (cf referring to Reference [3] (3GPP TS 33.401, V11.3.0, March 2012)). When the UE wants to handover from Relay Node to another eNB, there will be integrity protection failure for the un-synchronized (NH, NCC) pair between UE and network (cf referring to Reference [4] (3GPP TS 36.331 V9.10.0, March, 2012)).
But on the other hand, it is not necessary to synchronize the security key between UE and mobile relay node for every Relay Node handover. Because during Relay Node mobility procedure, actually the serving cell for the UE associated with the Relay Node does not change, so the security key (KeNB) stored in UE and mobile relay node is still synchronized. According to Reference [4], transmitting securityConfigHO to UE need one intra-eNB handover, which will cause a lot of RRC Reconfiguration messages transmitted in Uu interface, and a lot of unnecessary random access in Un port during Relay Node mobility. In the high speed public transportation scenario, one Relay Node installed on one high speed train may support more than 600 UEs (if every passenger of the train holds one LTE terminal), and the time interval between two times of relay node handover is about 5-18 seconds. If relay node needs trigger intra-eNB handover for every terminal during each relay node handover procedure, then the signaling cost over Uu interface for relay node handover will be too big, and that will impact the throughput of the relay node.